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Possible unconventional superconductivity in substituted BaFe2As2 revealed by magnetic pair-breaking studies.

Rosa PF, Adriano C, Garitezi TM, Piva MM, Mydeen K, Grant T, Fisk Z, Nicklas M, Urbano RR, Fernandes RM, Pagliuso PG - Sci Rep (2014)

Bottom Line: An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively.From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function.Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.

View Article: PubMed Central - PubMed

Affiliation: 1] Instituto de Física "Gleb Wataghin", UNICAMP, Campinas-SP, 13083-859, Brazil [2] University of California, Irvine, California 92697-4574, USA.

ABSTRACT
The possible existence of a sign-changing gap symmetry in BaFe2As2-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe1.9M0.1As2 (M = Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively. From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function. Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.

No MeSH data available.


Related in: MedlinePlus

Phase diagram for BaFe2−xMxAs2 (M = Co, Cu, and Ni) single crystals as a function of pressure.The dotted lines are guide to the eyes for the SC domes. The linear fit for the M = Cu compound (solid line) was obtained from the phenomenological expression ΔTc = S(S + 1)(a − bP). Using the same expression and S = 5/2, we obtain the dashed line for the M = Mn compound.
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f4: Phase diagram for BaFe2−xMxAs2 (M = Co, Cu, and Ni) single crystals as a function of pressure.The dotted lines are guide to the eyes for the SC domes. The linear fit for the M = Cu compound (solid line) was obtained from the phenomenological expression ΔTc = S(S + 1)(a − bP). Using the same expression and S = 5/2, we obtain the dashed line for the M = Mn compound.

Mentions: Finally, we comment on the effects of pressure on Tc, summarized in Fig. 4. For the Co and Ni substitutions, the rate dTc/dP is ~0.1 K/kbar and the application of pressure has little effect on Tc. Strikingly, this rate is three times larger for the M = Cu sample, while for M = Mn, no SC is observed. We argue that these results are linked to the magnetic pair-breaking discussed above. In particular, because pressure increases the hybridization between the Cu 3d bands and conduction electron bands, the copper bands become more itinerant, progressively losing their local moment character and consequently suppressing the magnetic IPB effect. Therefore, it is not surprising that the pure BaCu2As2 is a Pauli paramagnet with completely delocalized Cu 3d bands and no phase transition. Within this scenario, the fact that the Mn compounds do not display SC would follow from the fact that Mn2+ has a spin value five times larger than Cu2+. Interestingly, if the magnetic IPB mechanism is suppressed by pressure, Tc is, in principle, unconstrained to increase up to a maximum defined by the local distortions that the M-substitution creates. For Cu-substituted samples, it remains to be confirmed whether applying higher pressures with Diamond Anvil Pressure cells would further enhance or even suppress Tc in the impurity pair-breaking regime.


Possible unconventional superconductivity in substituted BaFe2As2 revealed by magnetic pair-breaking studies.

Rosa PF, Adriano C, Garitezi TM, Piva MM, Mydeen K, Grant T, Fisk Z, Nicklas M, Urbano RR, Fernandes RM, Pagliuso PG - Sci Rep (2014)

Phase diagram for BaFe2−xMxAs2 (M = Co, Cu, and Ni) single crystals as a function of pressure.The dotted lines are guide to the eyes for the SC domes. The linear fit for the M = Cu compound (solid line) was obtained from the phenomenological expression ΔTc = S(S + 1)(a − bP). Using the same expression and S = 5/2, we obtain the dashed line for the M = Mn compound.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4150111&req=5

f4: Phase diagram for BaFe2−xMxAs2 (M = Co, Cu, and Ni) single crystals as a function of pressure.The dotted lines are guide to the eyes for the SC domes. The linear fit for the M = Cu compound (solid line) was obtained from the phenomenological expression ΔTc = S(S + 1)(a − bP). Using the same expression and S = 5/2, we obtain the dashed line for the M = Mn compound.
Mentions: Finally, we comment on the effects of pressure on Tc, summarized in Fig. 4. For the Co and Ni substitutions, the rate dTc/dP is ~0.1 K/kbar and the application of pressure has little effect on Tc. Strikingly, this rate is three times larger for the M = Cu sample, while for M = Mn, no SC is observed. We argue that these results are linked to the magnetic pair-breaking discussed above. In particular, because pressure increases the hybridization between the Cu 3d bands and conduction electron bands, the copper bands become more itinerant, progressively losing their local moment character and consequently suppressing the magnetic IPB effect. Therefore, it is not surprising that the pure BaCu2As2 is a Pauli paramagnet with completely delocalized Cu 3d bands and no phase transition. Within this scenario, the fact that the Mn compounds do not display SC would follow from the fact that Mn2+ has a spin value five times larger than Cu2+. Interestingly, if the magnetic IPB mechanism is suppressed by pressure, Tc is, in principle, unconstrained to increase up to a maximum defined by the local distortions that the M-substitution creates. For Cu-substituted samples, it remains to be confirmed whether applying higher pressures with Diamond Anvil Pressure cells would further enhance or even suppress Tc in the impurity pair-breaking regime.

Bottom Line: An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively.From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function.Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.

View Article: PubMed Central - PubMed

Affiliation: 1] Instituto de Física "Gleb Wataghin", UNICAMP, Campinas-SP, 13083-859, Brazil [2] University of California, Irvine, California 92697-4574, USA.

ABSTRACT
The possible existence of a sign-changing gap symmetry in BaFe2As2-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe1.9M0.1As2 (M = Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively. From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function. Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.

No MeSH data available.


Related in: MedlinePlus